Along with cellular telephones, paging units are becoming a common radio communication consumer device. A conventional paging unit has the ability to receive paging signals that are transmitted simultaneously on a forward channel by a plurality of transmitters throughout a geographic area. The paging signals cause the paging unit to beep or provide some other indication to the user that a paging message has been received. This paging message may be alphanumeric data or in some instances may be a digitized voice signal. However, recent advancements in paging technology have included the production of so-called "two-way paging" units that not only receive paging signals but also transmit their own acknowledge signals on a reverse channel back to a central paging terminal. Typically, the acknowledge signal is transmitted from the paging unit to a receiver which relays the acknowledge signal to the central paging terminal, to determine whether the paging unit has received the paging message correctly.
In order to prevent two paging units from transmitting their acknowledge signals simultaneously on the reverse channel, prior art two-way paging terminals provide an indication to a paging unit of the time period during which the paging unit is to transmit its acknowledge signal. Once an acknowledge signal is received by the paging terminal, it is associated with a particular paging unit in order to determine whether the paging unit has received its paging message correctly. One way of accomplishing this is by determining whether the paging unit transmitted its acknowledge signal at the correct time. For example, if a paging unit is instructed to transmit its acknowledge signal at some predetermined time and an acknowledge signal is received at the predetermined time, then the paging system can assume that the paging message was correctly received by the proper paging unit.
In order to determine whether a paging unit has transmitted an acknowledge signal at the proper time, the receiver(s) of the two-way paging system must be synchronized to the transmitters that transmit the paging signals. Prior art paging systems have achieved this synchronization by designing receivers that monitor the forward channel for the paging system's transmissions as well as the reverse channel for the acknowledge signals from the paging units. However, such receivers are inefficient because they must include two .radio reception circuits, the first being tuned to transmissions from a paging transmitter and the second to acknowledge signals from the paging units. This not only increases the cost and complexity of a paging receiver, but increases the likelihood that a malfunction will occur.
As an alternative to synchronizing the paging receivers using the paging signals provided by the paging transmitters, it is possible to encode the time of transmission into the acknowledge signal transmitted from the paging unit to the receiver. However, such a system is inefficient because this adds unnecessary overhead to the reverse channel.
Therefore there is a need for a paging system that can synchronize multiple transmitters and receivers without requiring that the receivers monitor the paging system's message transmissions and without transmitting unnecessary information on the reverse channel.